The present invention relates to methods and apparatus for requesting system access on a random access channel in a communications system and, more particularly, to methods and apparatus for providing short random access channel frames for faster access request acknowledgment in a Universal Mobile Telecommunications System.
A major effort has been underway in the last decade to integrate multimedia capabilities into mobile communications. The International Telecommunications Union (ITU) and other organizations have been attempting to develop standards and recommendations that ensure that mobile communications of the future will be able to support multimedia applications with at least the same quality as existing fixed networks. Particularly, many global research projects have been sponsored in order to develop such next (third) generation mobile systems. Research and Development of Advanced Communication Technologies in Europe, RACE-1, and RACE-2, and Advanced Communications Technology and Services (ACTS) are examples of such efforts in Europe. It is known that in order to provide end users with the requisite service quality for multimedia communications, Internet access, video/picture transfer, high bit rate capabilities are required. Given such requirements, bearer capability targets for a third generation system have been defined as 384 kilobits per second (kb/s) for full coverage area and 2 Megabits per second (Mb/s) for local area coverage.
Universal Mobile Telecommunications System (UMTS) is a new radio access network based on 5 Megahertz Wideband Code Division Multiple Access (W-CDMA) and optimized for support of third generation services including multimedia-capable mobile communications. Since major design goals of UMTS are to provide a broadband multimedia communications system that integrates infrastructure for mobile and fixed communications and to offer, inter alia, the same range of services as provided by the fixed and wireless communications networks, UMTS must provide circuit-switched as well as packet-switched services, a variety of mixed-media traffic types, and bandwidth-on-demand. However, providing multimedia support implies the need for flexibility, that is, being able to support services with different bit rates and Eb/Norequirements, and to multiplex such services in a multiservice environment. UMTS is designed to be able to support such demands.
Referring to FIG. 1, an exemplary block diagram of a UMTS access network is shown. Particularly, a plurality of remote terminals 2 and 4 (e.g., mobile terminals) communicate with base stations (NODE-B) 6 via W-CDMA wireless links 8. The remote terminals may be a variety of devices such as a wireless phone 2 or a portable personal computer 4 with an internal or external modem. In the UMTS standard, a base station is called a NODE-B. These base stations communicate with a network component that provides radio resource management functions and is called a Radio Network Controller (RNC). Since UMTS is a W-CDMA system, soft handoffs are supported. In the case of soft handoffs, there are two base stations 6 serving one remote terminal. Thus, the remote terminal sends frames to these two base stations. When the two base stations receive the frames from the remote terminal, they send them to a Frame Selector Unit (FSU). The FSU decides which is a better frame, in terms of frame quality, to be sent to the core network. In UMTS, the FSU may be physically integrated with the RNC and as such, in FIG. 1, the RNC and FSU are shown as block 10, but also are separated functionally as block 12 (FSU) and block 14 (RNC). Other elements in the UMTS network perform conventional functions such as: the xLR databases 20, which provide home and visiting location information; and the interworking function (IWF) units. It is to be appreciated that the Universal Mobile Switching Center (UMSC) 16 serves as the mobile switching center for the base stations 6 in the UMTS. Sub-networks 18 are wireless service provider networks and CN1 through CNn are the core networks 24 to which the remote terminals are ultimately coupled.
Referring to FIG. 2, a diagram of the typical protocol stack in UMTS is shown. In UMTS, Layer 1 (L1) is the physical layer (PHY) which offers information transfer services to the MAC (Media Access Control) layer and higher layers. The physical layer transport services are described by how and with what characteristics data is transferred over the transport channels of the radio interface. Layer 2 (L2) is comprised of sublayers which include MAC, LAC (Link Access Control), and RLC and RLCxe2x80x2 (Radio Link Control). In UMTS, the functions performed in RLC are split and thus two RLC protocols (RLC and RLCxe2x80x2) are specified. The RLC and MAC layers provide real-time and non-real-time services. The MAC layer controls but does not carry out the multiplexing of data streams originating from different services. That is, the MAC layer, via logical channels, allows common physical communications channels (e.g., broadcast channel) to be shared by a number of remote terminals. IP (Internet Protocol) is the network layer.
xe2x80x9cUuxe2x80x9d refers to the UMTS-specific interface between a remote terminal and a base station, while xe2x80x9cIubxe2x80x9d refers to the UMTS-specific interface between a base station and the RNC/FSU. Layer 2 of the radio access network (i.e., left side of NODE-B on the protocol stack) is split into RLC and MAC layers, while Layer 2 of the core network (i.e., right side of NODE-B on the protocol stack) is more related to the technology used to transport network layer frames, e.g., ATM (Asynchronous Transfer Mode) or Frame Relay. IP is shown as the transport protocol, however, UNMTS is not so limited. That is, UMTS can cater to other transport protocols. Further details on the protocol layers may be found in Dahlman et al., xe2x80x9cLMTS/IMT-2000 Based on Wideband CDMA,xe2x80x9d IEEE Communications Magazine, pp. 70-80 (September 1998) and in ETSI SMG2/UMTS L2 and L3 Expert Group, xe2x80x9cMS-UTRAN Radio Interface Protocol Architecture; Stage 2,xe2x80x9d Tdoc SMG2 UMTS-L23 172/98 (September 1998).
One of the logical channels associated with the media access control (MAC) protocol of UTMS is the random access channel (RACH). RACH is an uplink common transport channel used to carry control information and short user packets from a remote terminal. Referring to FIG. 3A, a block diagram of an exemplary hardware implementation of a non-coherent RACH detection algorithm for use in a UMTS base station (NODE-B in FIG. 1) is shown. The RACH receiver 30 is capable of providing the following functions: detection, demodulation and decoding, and acknowledgement. The purpose of detection is to determine if a RACH burst (i.e., access request signal) is being sent by a remote terminal and to resolve the strongest multipath components of the incoming burst. The receiver 30 also demodulates and decodes the message contained within the corresponding RACH to ascertain the remote terminal identifier and the requested service. After decoding a remote terminal RACH transmission, the receiver generates an acknowledgement signal which the base station transmits to the remote terminal over a Forward Access Channel (FACH).
The RACH receiver 30 preferably performs the above functions in accordance with the following structure. A RACH transmission burst is received and demodulated by mixers 32 and then filtered in filters 34. The signal is then sampled in sampling unit 36. Despreader 38 decodes the signal in accordance with the spreading sequence, in this case, 512 Gold code. The decoded signal is buffered (buffer 40) and sent to time shifting unit 50. Also, the output of the despreader 38 is provided to integrator 42. The outputs of the integrator 42 are mixed (mixer 44) and provided to timing detector 46 and then threshold detector 48. The output of the threshold detector 48 indicates whether a valid signal was received from the remote terminal. This result is provided to time shifting unit 50. If it is a valid signal (e.g., above pre-determined threshold), the decoded signal is then down-sampled by unit 52. Then, depending on the preamble, described below, the signal passes through the 16 tap filter unit 54 to the preamble signature searcher 56. The output of the searcher 56 provides the base station with the encoded remote terminal""s identifier and information as to the service(s) requested by the remote terminal. The encoded information is then decoded by a convolutional decoder 58 and checked by a CRC (cyclical redundancy check) decoder 59.
Referring to FIG. 3B, a block diagram of an exemplary hardware implementation of an uplink transmitter 60 for use in a UMTS remote terminal (e.g., remote terminals 2 and 4) is shown. In a UMTS remote terminal, data modulation is dual channel QPSK (quaternary phase shift keying), that is, the I and Q channels are used as two independent BPSK (binary phase shift keying) channels. For the case of a single uplink DPDCH (dedicated physical data channel), the DPDCH and the DPCCH (dedicated physical control channel) are respectively spread by two different channelization codes (CC and CD) via mixers 62 and 64 and transmitted on the I and Q branches. The I and Q branches are multiplexed in IQ MUX 66. The total spread signal I+jQ is then complex scrambled by a connection-specific complex scrambling code in mixer 68. The real portion of the signal is then filtered in root-raised cosine filter 70, while the imaginary portion of the signal is filtered in root-raised cosine filter 72. The output of filter 70 is modulated in mixer 74 with a cos (xcfx89t) signal. The output of filter 72 is modulated in mixer 76 with a -sin (xcfx89t) signal. The two modulated signals are then added in adder 78. The composite signal is then amplified to a predetermined signal strength (i.e., power level) in amplifier 80 and then transmitted by an antenna (not shown). A control signal from a processor associated with the remote terminal fixes the power level of the signal to be transmitted. A similar arrangement may be used in the base station.
It is known that the physical RACH is designed based on a Slotted ALOHA approach. A remote terminal can transmit a random access burst 100 at eight well-defined time offsets (Access slot #1, . . . , Access slot #i, . . . , Access slot #8) relative to the frame boundary of the received broadcast control channel (BCCH) of the current cell, as illustrated in FIG. 4A. Each access slot is offset from the previous slot by 1.25 ms. As shown in FIG. 4B, the random access burst consists of two parts, a preamble part 102 of length 1 millisecond (ms), a message part 104 of length 10 ms, and an idle time 106 of length 0.25 ms in between the preamble part and the message part. There are a total of 16 different preamble signatures that are based on the Orthogonal Gold code set of length 16 (512 Gold code). The information on the available signatures and time offsets are broadcast on BCCH. Based on this structure, if the receiver has 128 (16 preamble signatures multiplied by 8 timeslots) parallel processing units, 128 random access attempts can be simultaneously detected. In other words, we have equivalent 128 random access channels for a maximum configured base station for the current cell. This arrangement is as per the current Layer 1 Expert Group specification in UTRAN/FDD Physical Layer Description Document, xe2x80x9cSMG2 UMTS Physical Layer Description FDD Part,xe2x80x9d Tdoc SMG2 UMTS-L1 221/98.
Referring to FIG. 4C, the existing RACH access slot structure is shown in which the frame structure (Frame 0, Frame 1, . . . , Frame n) is based on 10 milliseconds (ms). Also, it is assumed that the receiver requires a minimum of 2.5 ms to process an access burst. As shown, those remote terminals that have selected time offsets 0, 1, 2, 3, 4, and 5, can receive their MAC acknowledgements (from the base station) within 8.75 ms of their transmissions. That is, the maximum waiting period for an access burst (request signal), transmitted by a remote terminal within slots 0 through 5, is 8.75 ms. For example, Burst 0 is transmitted by a remote terminal at the start of Frame 0 and the remote terminal may receive an acknowledgement in response at the start of Frame 2, i.e., 8.75 ms later. Bursts 1 through 5 receive acknowledgements progressively sooner, up to Burst 5 which can receive an acknowledgement 2.5 ms after transmission. Acknowledgements generated by a base station for transmission in a given frame are typically grouped together in a common packet broadcast to the transmitting remote terminals.
However, as is evident, those terminals that have selected time offsets 6 and 7 can only receive their MAC layer acknowledgements within a maximum of 11.25 ms of their transmission, i.e., Burst 6 at 11.25 ms and Burst 7 at 10 ms. Again, this has to do with the fact that the minimum time to process an access request is assumed to be 2.5 ms. As such, access bursts 6 or 7 transmitted by remote terminals in Frame 1 extend beyond the 2.5 ms minimum processing period such that the base station cannot process the request and transmit acknowledgements in Frame 2. Thus, such remote terminals do not receive respective acknowledgements until Frame 3.
The present invention provides an improved RACH access burst arrangement and frame structure. That is, the invention provides methods and apparatus for supporting more than one access burst length in the UMTS access channel structure. Preferably, two access burst lengths are supported, e.g., 5 ms and 10 ms. Such an arrangement is advantageous in applications where it is beneficial to have fast access latency such as, for example, voice or other forms of real-time traffic. Also, the invention provides methods and apparatus for supporting multiple frame sizes. It is to be appreciated that further enhancement to access latency can be obtained by having the UMTS physical layer support multiple frame sizes. It is to be appreciated that the access burst signal transmitted by a remote terminal over the RACH may be an access request or data packets in the case where the RACH is being utilized for UMTS short message services.
In one aspect of the invention, apparatus for improving access latency in a random access channel in a communications system including at least one base station, comprises a remote terminal configured for selecting a time duration associated with an access signal (e.g., access request or data packets), the time duration being selected from among time durations which range from being substantially equivalent to a length of a transmission frame of the base station to being less than the length of the transmission frame. Preferably, the remote terminal may choose between an access burst duration with a message portion of about 10 ms and about 5 ms. The remote terminal then transmits the access signal having the selected time duration associated therewith to the base station over the random access channel in a selected time offset slot associated with the channel. Alternatively, the remote terminal may indicate to the base station, in advance of the access burst, the time duration it has selected.
In another aspect of the invention, apparatus for improving access latency in a random access channel in a communications system including at least one remote terminal, comprises a base station configured for selecting a transmission frame time duration associated with a random access channel, the transmission frame time duration being selected from among one or more supported time durations. Preferably, the base station may choose between a frame size of about 10 ms and about 5 ms. The base station is also configured for acknowledging a successful access signal transmitted by the remote terminal over the random access channel in a selected time offset slot associated with the channel. Alternatively, the base station may indicate to the remote terminal, in advance, the transmission frame time duration it has selected.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.